Nickel plated tape

ABSTRACT

An improved copper bump tape for tape automated bonding inhibits electromigration of the copper after bonding to a semiconductor device. The improved tape is characterized by the plating of a migration resistant metal onto the inner ends of connector beams of the tape. The migration resistant metal is coated onto all surfaces of the connector bump, except for the surface which is to be bonded to the semiconductor device. In this way, the surfaces of the bump which remain exposed after connection to the semiconductor are inhibited from electromigration.

This is a divisional of co-pending application Ser. No. 748,876, filedon June 26, 1985, now U.S. Pat. No. 4,707,418.

BACKGROUND OF THE INVENTION

The present invention relates generally to the connection or bonding ofintegrated circuit dies to metal lead frames prior to encapsulation ofthe dies. More particularly, the invention relates to tape automatedbonding of the dies to the lead frame with an improved copper tapetreated to resist electromigration.

In the packaging of semiconductor devices, it is necessary tointerconnect the device, generally referred to as a die or chip, to aplurality of electrical leads formed on a metal lead frame. Suchinterconnection is generally accomplished by one of two distincttechniques, referred to as wire bonding and tape automated bonding. Wirebonding is a sequential process where wire bonds are formed individuallybetween bonding pads on the die and the inner ends of the leads on theframe. While the wire bonding is the method of choice for someapplications, it is relative slow since it requires that the bonds beformed one after the other. Wire bonding a die can require severalseconds to complete even with the most rapid automatic wire bondingequipment.

Tape automated bonding, in contrast, allows the formation of allconnections between the die and the lead frame to be madesimultaneously. A metal tape, typically copper, is fabricated byconventional stamping and etching techniques so that a plurality ofindividual leads, often referred to as beams, are formed. The beams arecantilevered from the tape. That is, they remain connected to the tapeat their outer ends, while their inner ends are free to allow connectionto the die. By forming the inner ends of the beams in a pattern whichcorresponds precisely to the pattern of metallization pads formed on thedie, the inner ends of the beams can be aligned with the die and thenconnected in a single compression and heating step. In a similaroperation, the outer ends of the beams are connected to the lead frames,and the beams then severed from the remainder of the tape.

In connecting the inner ends of the lead beams to the semiconductor die,it is necessary to form a bump on either the die or the beam. The bump,which is formed from metal, provides both the material necessary forforming the connection between the beam and the die, as well asproviding for a short distance or offset between the beam and the die.There are advantages and disadvantages associated with forming the bumpon either the semiconductor die or the beam. The present invention isdirected at bumped bonding tapes, that is bonding tapes having aconnection bump formed at their inner ends.

It has been found by the inventors herein that when employing coppertape for tape automated bonding, problems can arise withelectromigration of the copper on the surface of the die. Suchelectromigration results from the potential difference between themetallization pads on the die which results in dendritic flow of thecopper on the die surface. In the worst case, the copper can migrate andform a short between two or more of the metallization pads, renderingthe device defective.

It would thus be desirable to provide an improved copper bonding tapewhich would avoid the problems associated with copper migration on thesurface of a die. It would be particularly desirable if the method forforming the improved tape would require only small departures fromexisting techniques for copper tape fabrication.

SUMMARY OF THE INVENTION

The present invention provides for an improved highly reliable copperbump tape wherein all surfaces of the bump, other than that surfacewhich bonds to a metallization pad on a semiconductor device, are coatedwith a thin layer of a migration resistant metal, typically nickel or anickel alloy. The migration resistant layer prevents electromigration ofthe copper caused by the potential difference between variousmetallization pads on the die surface. The metal layer also enhances thestrength of the tape as well as inhibiting corrosion of the tape duringsubsequent processing steps, yielding a highly reliable final product.The method of the present invention utilized to fabricate such tape isparticularly advantageous since it requires the addition of only asingle processing step to conventional tape fabrication processes.

The method of the present invention is utilized with single layer coppertape which are formed with connector bumps at the inner ends ofconnector beams. The connector bumps are formed in the connector beamsby conventional photoresist processing techniques. First, both sides ofthe copper tape are coated with photoresist, and the photoresist ispatterned to form bumps of the desired dimensions. After developing thephotoresist, and etching the copper to form the bumps, all surfaces ofthe bump are exposed except for a flat connecting surface which will bejoined to the metallization pad on the die. A nickel layer is thenapplied to the tape, and deposits on all surfaces of the bump other thanthe bonding surface. Thus, after the photoresist is stripped, the bumpson the copper tape are coated on all exposed surfaces except for thatwhich will be joined to the semiconductor die.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a copper bonding tape prior toprocessing by the method of the present invention.

FIG. 2A is a cross-sectional view of an individual connector beam havinga layer of photoresist deposited on both sides.

FIG. 2B is a right-end view of the coated connector beam of FIG. 2A.

FIG. 3A is a cross-sectional view illustrating the connector beam ofFIG. 2A, with the photoresist patterned and developed to expose thesurfaces of the connector beam which are to be etched to form theconnector bump.

FIG. 3B is a right-end view of the connector beam of FIG. 3A.

FIG. 4A is a cross-sectional view of the connector beam of FIG. 3A,illustrated after etching.

FIG. 4B is a right-end view of the connector beam of FIG. 4A.

FIG. 5A is a cross-sectional view of the connector beam of FIG. 4A,illustrated after a migration resistant layer has been deposited on theexposed surfaces.

FIG. 5B is a right-end view of the connector beam of FIG. 5A.

FIG. 6A is a cross-sectional view of the connector beam of FIG. 5A,shown after the photoresist has been stripped.

FIG. 6B is a right-end view of the connector beam of FIG. 6A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a single layer copper tape 10 of the type which maybe utilized in the present invention. The tape 10 includes a pluralityof individual frames 12, each of which include a number of individualconnector beams 14. The connector beams are formed by punching the tapeso that the beams 14 are conneted at their outer ends 16 to the tape,while their inner ends 18 are suspended freely about the central openingof the frame 12. By properly positioning the inner ends 18 of the beams14, the pattern of the inner ends will correspond to the pattern ofbonding pads on a particular semiconductor die (not illustrated).

The method of the present invention provides for the formation ofconnector bumps at the inner ends 18 of the beams 14. During theformation of the bumps, as will be described in detail hereinafter, amigration-resistant metal layer is applied to all surfaces of the bumpsother than the surface which is eventually bonded to the bonding pads onthe semiconductor device. Such a metal coating prevents electromigrationof the copper when it is bonded to the semiconductor device, as well asinhibiting corrosion and strengthening the connector beams.

Referring now to FIGS. 2-6, the processing method of the presentinvention will be described in detail. After proper cleaning of thecopper tape 10, a photoresist layer 20 is applied to the tape 10 in aconventional manner. The photoresist 20 on an individual connector beam14 is illustrated in FIGS. 2A and 2B.

The photoresist layer 20 is then patterned and developed to form thepattern as illustrated in FIGS. 3A and 3B. The connector beam 14 isexposed on three surfaces 22, 24, and 26 at its inner end 18. A fourthsurface 28 (spaced away from the inner end 18) is also exposed.

The construct illustrated in FIGS. 3A and 3B is then exposed to ananisotropic etching process where exposed horizontal surfaces, such assurface 28, are preferentially etched relative to substantially verticalsurfaces such as 22, 24, and 26. References to horizontal and verticalwill be made relative to the drawings, and it will be appreciated thatin practice the tape may be oriented in any preselected direction solong as the direction of etching is oriented to preferentially etchsurface 28.

The result of the anisotropic etching is illustrated in FIGS. 4A and 4B.A cavity 30 is formed in the exposed area 28, while the exposed surfaces22, 24, and 26 are subtantially unchanged. As a result of the formationof cavity 30, a connector bump 32 is formed at the inner end 18 of theconnector beam 14. The bump 32 includes four exposed surfaces 22, 24,26, and 34. The remaining surface of the bump is covered by thephotoresist layer 20a. It is this covered surface which will eventuallybe bonded to the metallization pad on the semiconductor device.

Referring now to FIGS. 5A and 5B, a non-migrating metal layer 36 isplated onto the exposed areas of the connector beams 14. Suitablemigration-resistant metals include nickel, nickel alloys, chromium,titanium/tungsten, palladium, gold, and the like. Preferred is thenickel coating because it is generally less expensive. The metal layer36 may be deposited by conventional electroplating techniques. Thethickness of the migration resistant metal layer will be in the range ofabout 0.1 micron to 10 microns, usually being about 0.25 micron.

Referring now to FIGS. 6A and 6B, the photoresist layer 20 is stripped,leaving the connector bump 32 protected by the metal coating 36 on fourside surfaces 22, 24, 26, and 32 adjacent a bonding surface 38 on thebottom bonding. The bonding surface 38, which is free from the nickelcoating, is intended to be bonded to the metallization pads on thesemiconductor device by conventional techniques. After such bonding, thenickel layers on the adjacent surfaces of the bump 32 will inhibitelectromigration of the copper on the surface of the semiconductor.

The remaining processing steps in the tape automated bonding of thesemiconductor to the copper tape are conventional and need not bedescribed here.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

What is claimed is:
 1. A process for fabricating copper bump tape, saidprocess comprising:applying a photoresist layer to both sides of thetape; exposing the photoresist to form a pattern which defines a bump onthe inner ends of connector beams on the tape; developing thephotoresist to expose the inner ends of the connector beams, except fora surface which defines the bump; etching the copper tape to form thebump; plating a layer of a migration resistant metal onto the copperwhile the photoresist remains in place, whereby the exposed surfaces ofthe bump are coated with the migration resistant metal, but the surfaceprotected by photoresist is not coated; and removing the remainingphotoresist.
 2. A process as in claim 1, wherein the migration resistantmetal is nickel or a nickel alloy.
 3. A process as in claim 1, whereinthe migration resistant metal is plated to a thickness in the range of0.1μ to 10μ.
 4. A process as in claim 3, wherein the thickness is 0.25μ.